The short answer
Immune checkpoint inhibitors are a type of immunotherapy that blocks the off signals that keep your immune system from attacking cancer. By blocking checkpoint proteins, they free T cells to kill cancer cells. They treat many cancers but can also cause inflammation as a side effect.
Immune checkpoints are a normal part of the immune system that keeps immune responses from harming healthy cells.
Some cancers use checkpoints to send an off signal that stops T cells from attacking them.
Checkpoint inhibitors block these off signals, letting T cells kill cancer cells.
They are approved to treat many cancer types, including melanoma, lung, and bladder cancer.
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The full explanation.
The simple version
Your immune system has built-in brakes. They are called immune checkpoints, and they are a normal part of the system. Their job is to keep an immune response from getting so strong that it destroys healthy cells.
The problem is that some cancers learn to use those brakes to protect themselves. Immune checkpoint inhibitors are a type of immunotherapy. They release the brakes, so your immune system can fight the cancer.
Checkpoint inhibitors take the brakes off your immune system so it can attack cancer.
How checkpoints normally work
Immune cells called T cells carry proteins on their surface. Those proteins can bind to partner proteins on other cells, including some tumor cells. Together they are called immune checkpoint proteins.
When a checkpoint protein binds its partner, the pair sends an off signal to the T cell. In healthy tissue that off signal protects you. It stops the immune system from attacking your own cells. But a tumor can send the same off signal. Then the immune system does not destroy the cancer.
How the drugs help
Immune checkpoint inhibitors block checkpoint proteins from binding their partner proteins. The off signal never gets sent. That leaves the T cells free to kill cancer cells.
Different drugs target different checkpoints:
- One type acts against a checkpoint protein called CTLA-4.
- Others act against a checkpoint protein called PD-1, or its partner protein PD-L1.
Some tumors make a lot of PD-L1 to turn down the T cell response. Blocking PD-1 or PD-L1 can free the T cells to attack.
Which cancers they treat
Checkpoint inhibitors are approved to treat some people with many cancer types. They include:
- breast cancer
- bladder cancer
- cervical cancer
- colon cancer
- head and neck cancer
- Hodgkin lymphoma
- liver cancer
- lung cancer
- renal cell cancer, a type of kidney cancer
- skin cancer, including melanoma
- stomach cancer
- rectal cancer
- any solid tumor that cannot repair certain errors in its DNA
Side effects to know about
Checkpoint inhibitors affect people in different ways. What you get depends on several things. How healthy you are before treatment. Your type of cancer, and how advanced it is. The drug you receive, and the dose. Doctors and nurses cannot know for sure whether side effects will happen, when, or how serious they will be. So it helps to know the signs to watch for.
Common side effects include:
- rash
- diarrhea
- fatigue
Rarer side effects can include inflammation that spreads. What it looks like depends on the organ involved. The skin can change and itch. The lungs can cause cough and chest pain. The colon can cause belly pain and diarrhea. The pancreas can lead to diabetes. The liver can develop hepatitis. Problems can also show up in the pituitary gland, heart muscle, kidneys, thyroid, or nervous system.
Because side effects can involve inflammation in different organs, report new symptoms to your team quickly.
When to get help sooner
- Call 911 or go to an emergency department if you have chest pain, a pounding or racing heart, or you are fighting for breath. NCI lists myocarditis and lung inflammation among the rarer effects of these drugs.
- Call your care team the same day if a new cough starts, stairs leave you short of breath, or belly pain comes with watery stools. Colon inflammation can move quickly. The same applies to a temperature of 100.4°F (38°C) or higher.
- Call your care team within a day or two if a rash spreads, itching keeps you awake, your eyes or skin look yellow, or you feel new muscle weakness or numbness. These can be early signs of inflammation in the skin, liver or nerves.
Words to know
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Common questions
What are immune checkpoints?
Immune checkpoints are a normal part of the immune system. Their role is to prevent an immune response from being so strong that it destroys healthy cells. They work when proteins on immune cells called T cells bind to partner proteins on other cells, sending an off signal.
How do checkpoint inhibitors work?
Some tumors use checkpoints to send an off signal that stops T cells from attacking them. Checkpoint inhibitors block checkpoint proteins from binding with their partner proteins. This prevents the off signal from being sent, which allows the T cells to kill cancer cells.
Which checkpoint proteins do these drugs target?
One type of drug acts against a checkpoint protein called CTLA-4. Others act against a checkpoint protein called PD-1 or its partner protein PD-L1. Some tumors turn down the T cell response by making a lot of PD-L1.
Which cancers are treated with checkpoint inhibitors?
They are approved to treat many cancers, including breast, bladder, cervical, colon, head and neck, liver, lung, kidney (renal cell), stomach, and rectal cancers, Hodgkin lymphoma, and skin cancers such as melanoma. They can also treat any solid tumor that cannot repair certain DNA errors.
What side effects can checkpoint inhibitors cause?
Common side effects include rash, diarrhea, and fatigue. Rarer side effects involve widespread inflammation. Depending on the organ affected, inflammation can cause problems in the skin, lungs, colon, liver, kidneys, heart, thyroid, pancreas, or nervous system.
Can doctors predict my side effects?
No. Doctors and nurses cannot know for sure when or if side effects will occur or how serious they will be. That is why it is important to know which signs to look for and what to do if they happen.
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Sources last checked: 2026-08-11 what this meansLast updated: 2026-08-11Next planned review: 2027-01-14
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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